Programmable charge pump device

ABSTRACT

The invention relates to charge pump device and a display driver with a charge pump device. Further it relates to a display module with a display driver using a charge pump device and a telecom terminal having such a display module. To provide a device which needs a charge pump for generating a higher voltage as the supply voltage a charge pump device is proposed containing at least two stages (S), whereby a stage (S) comprises a switch (SW n ) and a charge device (CS n ) which are arranged to generate a voltage higher than the supply voltage (V dd ), whereby the stages (S) are arranged in serie and a required multiplication factor (MF) of the charge pump (CP) is adjustable by activating/deactivating a definable number of stages (S), whereby the switches (SW n ) of each stage (S n ) are arranged in the same way. Thereby it will achieved a freely programmable multiplication factor of the supply voltage and in the same time having switches in the charge pump which are optimized in the power consumption by programming a multiplication factor smaller than the maximum multiplication factor.

[0001] The invention relates to charge pump device and a display driverwith a charge pump device. Further it relates to a display module with adisplay driver using a charge pump device and a telecom terminal havingsuch a display module.

[0002] Charge pump devices or voltage multipliers are used in devices,where a higher voltage than the supply voltages is necessary. Thesecharge pump devices will be used especially in LCD modules and in flashmemories, because the highest voltage required is larger than thevoltage supply available. These parts are nowadays produced in largequantities and the current efficiency is a major issue. They areintended to be used in battery-operated applications, so low currentconsumption and recently low voltage become more and more important.

[0003] The LCD modules find a large application in cellular phones andother hand-held devices like organizers, laptops, PDAs, etc. In thephones, the supply voltage available for analog blocks is 2.8V and a LCDgraphic display can be driven with voltages of 6 to 16V. The voltageneeds to be multiplied to at least 3 times of the supply voltage.Because a display should be able to operate within a large range of thesupply voltage, the multiplication factor should vary with theapplication.

[0004] A charge pump is composed by a cascade of several stages, wherebya stage contains at least a switch or a diode, a charge storing element,mostly realized as a capacitor and one driver. The driver commands thecharge storing elements and is operated by periodical signals or phases.A charge stored in the first stage will be forwarded to the next stage,where it will be added to the charge of this stage, so a higher voltagewill be generated, which is provided to the devices i.e. an LCD Modulein a mobile phone.

[0005] There are two kinds of charge pumps:

[0006] with on-chip capacitors or

[0007] with external capacitors.

[0008] The on-chip capacitors driver is the simplest solution forChip-On-Glass realizations and can offer a current efficiency of about95%. The costs for the one-chip solution are lower. The charge pump withexternal capacitors is more efficient, but less suited to theChip-On-Glass solutions because it is more expensive.

[0009] In order to change the multiplication factor of a charge pump toadapt the solution to different applications it would be possible to usenot all stages of the charge pump to generate output voltages below themaximal possible voltages, where all stages of the charge pump are used.For example, one of the characteristics of the liquid crystal of thedisplay is that it must be driven with a higher voltage at lowtemperatures, while the battery is only able to supply a low voltage. Inthis case, selecting a higher multiplication factor than at roomtemperature could improve the current consumption of the display module.

[0010] In the present case, even with not so low supply voltage V_(dd),when not all the stages are used, it happens that one of the middlestages is used as the first switching stage, bringing a currentefficiency loss.

[0011] So the charge pump is functional but it consumes more power thanthe optimum, because of the back current. To minimize the losses due tothe back current, which is flowing from one stage to the stage beforethat stage towards the input of the charge pump device, while the switchin the looked stage is not yet completely open, the switches have beenimproved.

[0012] To solve the problem with the back current the switches of thefirst stage and the last stage of the charge pump are different from themiddle switches.

[0013] In known applications, the charge pump has been designedincluding a mask change, with which the first used stage was transformedto increase efficiency, but forbidding further software changes of themultiplication factor. This solution is quite restrictive.

[0014] The EP 03190634 describes a voltage multiplier circuit comprisinga serie of rectifier elements, which are alternately rendered conductiveby alternatively applying complementary clock signals to capacitanceswhich are connected to junction points of pairs of neighboring elements.

[0015] In a programmable charge pump, the multiplication factor isdetermined by how many stages are switching. The stages closer to theinput, beginning with the first stage, are turned conductive to bringthe supply voltage at the input of the first switching stage. Theintermediate or middle stages are all of the type in FIG. 2, withbootstrap capacitor to increase the level of the command gate.

[0016] In a programmable charge pump, the first switching stage canthough be any one of the stage.

[0017] By using that kind of stages or elements as a first stage of avoltage multiplier or a charge pump the input is constant (Vdd) and cannot be switched as it would be a middle stage. In this case, the switchcan not be turned ON if the supply voltage is lower than 2|Vtp| (Vtp isthe threshold voltage of the PMOS).

[0018] As the supply voltage V_(dd) becomes lower and lower, thisproblem arose. The switches used in nowadays charge pumps are built withPMOS transistors as switches of the stage. To improve the behavior ofthose switches, either bootstrap capacitors or level shifters are usedto drive the gate of a PMOS switch. Bootstrap capacitor technique forall the middle and the last stage is used and recognized as a veryefficient solution. But the switches can operate correctly only for asupply voltage V_(dd)>2|V_(tp)|, where V_(tp) is the threshold voltageof a PMOS transistor. The limitation is in case a switch with bootstrapcapacitor which is used as first stage and supplied between V_(dd) andV_(ss) permanently, whereby V_(ss) is ground of the switch, assumed as0V, but it can vary because of parasitic resistances. For lower supplyvoltage, the charge pump will not work. A |V_(tp)| can be as high as1.3V, varying with the process parameters and the temperature.Therefore, any V_(dd) lower than 2.6V will cause a problem.

[0019] So it is an object of the invention to provide a device with acharge pump with a freely adjustable multiplication factor and minimizedpower losses.

[0020] The object will be solved with a charge pump device containing atleast two stages, whereby a stage comprises a switch and a charge devicewhich are arranged to generate a certain voltage higher than the supplyvoltage, whereby the stages are arranged in serie and a requiredmultiplication factor of the charge pump is adjustable byactivating/deactivating a certain number of stages and the switches ofeach stage are arranged in the same way.

[0021] The invention deals with that limitation of the low supplyvoltage and also constitutes a solution to efficiency loss when themultiplication factor is programmed lower than the maximum. Theimprovement concerns the switches with bootstrap capacitors. The newcharge pump is functional beginning with V_(dd)=|V_(tp)| and evenprogramming a different multiplication factor than the number of stagesN is not reducing the current efficiency.

[0022] An embodiment of a charge pump device according to that inventionis characterized in that for a adjusting a multiplication factor smallerthan the maximal possible factor the stages beginning from an input ofthe charge pump device will be deactivated. By this the possibility forusing this charge pump for different application is assured, whereby dueto the switching off of not needed stages no power loss appears.

[0023] So for applications where not the maximal multiplication factoris needed, the not used stages can be switched off without any the lossof power.

[0024] To achieve this the charge pump is composed of a cascade ofswitches, drivers and charge devices, realized as stage capacitors. Eachswitch SW of a stage comprises a switch MP1 which is arranged between aninput IN and an output OUT of the stage (S) of the charge pump device,further two transistors MP2 and MP3 for controlling the isolated bulk ofthe switch MP1 and a fourth transistor MP4 to charge a boot capacitor(C_(B)), whereby the boot capacitor (C_(B)) is arranged for storing thecharge to drive the gate of the switch MP1 further comprises a gateswitch control unit GSU, whereby the gate switch control unit GSU isarranged to switch the gate of the switch MP1.

[0025] PMOS transistors have isolated bulk because they are built in aN-well area. By biasing this N-well area always to the highest potentialthe junction is reverse biased and isolates the PMOS transistorelectrically from the substrate. The role of MP2 and MP3 is to determinewhich is the highest potential between the input and the output. All ofthe PMOS transistors of one switch are built in the same N-well area. Inan embodiment of that invention the switch MP1 is preferably realized asisolated bulk transistor.

[0026] In an embodiment of that invention the charge pump devicecontaining a level generation unit (LGU) for providing control signalsfor the gate switch control unit (GSU), whereby the gate switch controlunit is foreseen to connect or disconnect the gate of the switch MP1transistor from the C_(B) capacitor.

[0027] In an preferred embodiment of the invention a charge pump devicehas a gate switch control unit, which is arranged to provide controlsignals to switch MP1 of the stage in case of voltages below V_(dd).

[0028] By using the gate switch control unit GSU and the levelgeneration unit LGU it is achieved to disconnect the bootstrap capacitorC_(b) from the gate of MP1, when MP1 needs to be turned ON and drivingdirectly with V_(dd). When it needs to turn MP1 OFF, MN1 become OFF andMP5 is ON. The other two transistors MN2 and MP6 are to commandcorrectly the gate of MP5. In the inventive solution the two transistorsMN1 and MP5 located in the GSU are not conducting in the same time. MN1is active when the switch MP1 has to be turned conductive: we apply 0V(alias <switchb>) to the gate of MP1 instead of the bottom plate of thebootstrap capacitor C_(b) that can be |Vtp| in that moment and then itwill be disconnected by the MP5, not conducting because its gate is tothe selected highest potential.

[0029] So with that new architecture it is possible to realize thecharge pump with switches having bootstrap capacitors in all stages andadditionally the charge pump will be working at V_(dd)=|V_(tp)|.

[0030] Through it all stages can be build or realized in the same way,which leads to the results that by programming a multiplication factorlower than the maximal one, one of the middle stages of the charge pumpcan be used as first stage by avoiding the loss of power.

[0031] Further by using that kind of stages of a charge pump device itis possible to reprogram a multiplication factor after using a certainfirst multiplication factor.

[0032] The object will also be solved by a display driver for providingdisplay information and voltages to a display unit with a charge pumpdevice containing at least two stages, whereby a stage comprises aswitch and a charge device which are arranged to generate a voltagehigher than the supply voltage, whereby the stages are arranged in serieand all stages are realized in the same way and a requiredmultiplication factor is adjustable by activating/deactivating a certainnumber of stages.

[0033] Further the object will be solved by an display module havingdisplay unit and a display driver with a charge pump device, containingat least two stages, whereby a stage comprises a switch and a chargedevice which are arranged to generate a voltage higher than the supplyvoltage, whereby the stages are arranged in serie and all stages arerealized in the same way and a required multiplication factor isadjustable by activating/deactivating a certain number of stages.

[0034] The object will also be solved by an Telecom terminal having adisplay module (DM), a display unit (DU) and a display driver (DD) witha charge pump (CP) device, containing at least two stages (S), whereby astage comprises a switch (SW_(n)) and a charge device (CS_(n)) which arearranged to generate a voltage higher than the supply voltage (V_(dd)),whereby the stages (S) are arranged in serie and all stages (S) arerealized in the same way and a required multiplication factor (MF) isadjustable by activating/deactivating a certain number of stages (S).

[0035] In battery operated telecom terminals it is very important tohave an LCD module with a minimized power consumption, due to a higherStandby or operating time.

[0036] The invention will be described in detail hereinafter withreference to a drawing; therein:

[0037]FIG. 1 shows an example of a conventional charge pump

[0038]FIG. 2 shows a stage of a conventional charge pump

[0039]FIG. 3 shows a stage with gate switch control unit and levelgeneration unit

[0040]FIG. 4 shows a LCD Module with a Chip-On-Glass display driver

[0041]FIG. 1 shows a schematic of a conventional charge pump CP. Thereare several stages S_(n) A stage S_(n) is composed of a switch SW_(n)and charge storing element or device CS_(n) and a driver D. The driversD drive the charge storing elements CS₁-CS_(n) alternatively in pairsaccording to a periodically signal. The C_(Buffer) is not on the chip,due its size and is arranged to smooth out the output voltage. Thesupply voltage V_(dd) is provided at the input IN of the charge pump CP.The higher voltage for providing i.e. a LCD module is generated at theOutput OUT of the charge pump device CP.

[0042] The above described problem occurs when a switch SW_(n) withbootstrap capacitor C_(B) is used as the first stage, supplied at theinput with V_(dd). The FIG. 2 shows such a switch SW_(n) used in acommon charge pump CP.

[0043] The transistor MP1 is the main switching element of the switchSW_(n), with two states: In the ON or closed state it lets the currentflow form drain to source, in the OFF or open state, no current flowsthrough. MP2 and MP3 form a voltage comparator. Their role is to biasthe n-well area or the isolated bulk of the switch MP1 with the highestvoltage, to avoid leakage due to forward biased diodes. C_(B) is muchsmaller than the stage capacitor CS_(n). Its role is to provide enoughvoltage to open the switch MP1 in one of the two phases of the pumping.The transistor MP4 plays a role in turning off the MP1 and in providinga DC path to charge the node switchbhv (bootstrap capacitor) when MP1 isOFF, from the stage capacitor CS_(n).

[0044] In the first phase the switch SW_(n) as represented in the FIG. 2is programmed as the first active switch SW₁ and is initially ON. C_(B)is initially discharged. The input IN is set to the supply VoltageV_(dd), the bottom plate of CS₁ to V_(ss)=0V and the Output OUT ischarged to V_(dd). MP1 and MP2 are ON, MP3 and MP4 are OFF,WELL=IN=V_(dd). C_(B) is discharged and switchbhv is isolated,switchb=0V so C_(B) remains discharged and pulls switchbhv to 0V,keeping MP1 and MP2 ON.

[0045] In the next phase, the switch SW₁ needs to be turned off beforethe bottom plate of the CS₁ is driven to V_(dd). The following eventstake place: IN=V_(dd) (because it is the first stage). The node switchbis driven to V_(dd). Because C_(B) had no DC path and was not driven,the switchbhv node is pulled up by the bootstrap capacitor C_(B). MP1and MP2 are turned OFF. MP3 and MP4 remain also OFF. Then, the bottomplate of the stage capacitor CS₁ is driven from 0V to V_(dd), and thenext stage S₂ switch SW₂ is turned ON. The output node OUT will have2V_(dd) and will discharge in the next stage capacitor CS₂. BecauseV_(dd)>|V_(tp)|, MP3 and MP4 will turn ON. Through them, the nodes WELLand switchbhv will have the same voltage as the output node OUT untilthe output OUT voltage drops to V_(dd)+|V_(tp)|. Then, MP3 and MP4 willturn OFF. At this point, the bootstrap capacitor C_(B) is charged withV_(dd)+|V_(tp)|−V_(dd)=|V_(tp)| (with switchbhv>switchb). MP1 and MP2stay OFF because their gate voltage is higher than the drain and thesource voltages. The output OUT can decrease under V_(dd)+|V_(tp)| intwo cases: at the beginning when all capacitors are discharged and whena large current is required at the output.

[0046] Following, the next stage S₂ switch SW₂ is turning off, thebottom plate of the stage capacitor CS₂ is again driven to 0V. Theoutput node OUT is now lower than |V_(tp)|, IN=V_(dd), switchb is tiedto 0V. MP3 and MP4 continue to be OFF and we need to turn ON again MP1and MP2. This is only possible when switchbhv<IN −|V_(tp)|. If this wasNOT the first stage S₁, there would at the input IN 2 times V_(dd) fromthe previous stage, so there will be no problem to meet the requirement.But as a first stage is IN=V_(dd).

[0047] But C_(B) was charged with |V_(tp)| and that now the switchbhvnode has no DC path to discharge this capacitor. The node switchb is 0V(it has to turned ON), so switchbhv is pulled down to |V_(tp)|.

[0048] Until now, the circuit worked because switchbhv=|V_(tp)|<IN−|V_(tp)|, so V_(dd)>2|V_(tp)|, which is the limit of this architecture.

[0049] To overcome this, the first stage S₁ must not have a bootstrapcapacitor C_(B), or this could be shorted with a mask change, sodefinitively only for the one application. Because V_(dd)=2.8V is notmuch larger than 2|V_(tp)|=2.6V in the worst case, there is also a lossof efficiency.

[0050] New LCD drivers need to be operated from lower supply voltagesthan this limit of 2.6V, and for flexibility the maximum V_(dd) could beas high as 4.5V. Programming the multiplication factor is a request forall new display drivers. Therefore, an architecture of the switchesallowing lower input voltages is required. This invention is a solutionto both the low V_(dd) voltage and to the loss of efficiency in the sametime.

[0051] Improved Architecture of the Middle Switches

[0052] In the FIG. 3, the new switch architecture with bootstrapcapacitor is illustrated. A gate switch control unit GSU has been addedand a level generation unit LGU. They are rather small in comparisonwith MP1 and MP4, and comparable in size with MP2 and MP3.

[0053] The GSU contains two transistors MN1 and MP5. The MN1 is arrangedbetween the switchb and the switchbhv node, whereas the MP5 is locatedbetween swoffhv and the switchbhv. The LGU contains two transistors MP6and MN2 to generate the signal pgatectrl. Additionally the LGU containsa inverter INV to generate the signal switch2 from the signal switchb.

[0054] The idea is: if it would possible to turn ON MP1 always with 0Vand turn it OFF with a voltage always>V_(dd)+|V_(tp)| in the otherphase.

[0055] For turning ON the MP1, MN1 is active: switchb is 0V, switch2 isV_(dd), so MN1 is ON and pulls the gates of MP1 and MP2 to 0V. Forturning OFF, the node switchb comes to V_(dd), switch2 to 0V so MN1 isOFF, MN2 is ON, WELL−swoffhv<|V_(tp)| so that MP6 is OFF. This ties thenode pgatectrl to 0V, turning ON the MP5. Consequently, the nodeswitchbhv is disconnected from switchb and connected to swoffhv. Asbefore, when MP1 is OFF, swoffhv is charged initially with 2Vdd anddecrease because of the load downto as low as Vdd+|Vtp|.

[0056] In the next phase, the node switchb is driven to 0 to turn ON.This turns strongly MN2 OFF. The node OUT is lower than |Vtp|. The nodeswoffhv is now |Vtp|, and switch2 is V_(dd). MN1 is therefore driven ONand switchbhv goes to 0V quickly. MP5 is still open in a certainmeasure: swoffhv=|Vtp|, pgatectrl is no more driven strongly to 0V butfloating (WELL is equal to IN because MP2 switched ON). MP6 is in weakinversion, slightly conducting. It drives pgatectrl at the beginningweak towards WELL=IN=V_(dd). In the same time C_(B) is dischargedtowards 0V by MP5, still ON. Because of this discharge, and becauseinitially swoffhv has |Vtp|, MP5 will turn off very quickly as pgatectrlis no more driven strongly to 0V, and from this moment C_(B) does notloose any more charge. This solves the problem and the switch functionin theory until V_(dd) equals one |V_(tp)|.

[0057] When the new switch is not used as first active stage but asmiddle stage, i.e. as a second stage, the IN voltage is between 2V_(dd)and |V_(tp)| (from the same considerations as before) and when theswitch is ON, WELL is 2V_(dd), swoffhv is V_(dd)+|V_(tp)|, anddischarges through MP5 until it reaches WELL−|V_(tp)| (see MP6 whichturns strongly ON), so from V_(dd)+|V_(tp)| to 2V_(dd)−|V_(tp)|. Theamount of charge lost for C_(B) (this is taken in the other phase fromCS) per cycle is thenC_(B)*(V_(dd)+|V_(tp)|−2V_(dd)+|V_(tp)|)=C_(B)*(2|V_(tp)|−V_(dd)). IfV_(dd)>2|V_(tp)|, no charge is lost, so the efficiency is not degraded.

[0058] An application for low voltage operation of a voltage multiplieror a charge pump device for on-chip stage capacitors is shown in FIG. 4.The charge pump CP is a very important part of every LCD driver IC (DD),since the available supply voltage V_(dd) is always lower than what isrequired to have good optical performance of the LCD module DM. Thecharge pump CP current efficiency is determining the low currentconsumption of the whole LCD module DM. In the FIG. 4 a LCD module DMwith a Chip-On-Glass display driver (DD)-IC is depicted. The charge pumpdevice CP is provided with the supply voltage Vdd. Additionally it isconnected with a not shown Programming interface PI. By using theprogramming interface PI a multiplication factor MF can be chosen, whichis small than then the maximal possible MF_(max). This is achieved bydeactivating several stages S. This solution allows to change the MF independency of the application, whereby the charge pump with the changedMF can be reprogrammed to an other MF without restrictions.

[0059] The LCD modules are important parts of every mobile phone andother hand-held devices. They are manufactured in large volumes. Anotherapplication could be flash memories, which also need operating voltageslarger than the available supply. Flash memories are used for example indigital still cameras, as storage element, and low power and low voltageis also required.

1. Charge pump device containing at least two stages (S), whereby astage (S) comprises a switch (SW_(n)) and a charge device (CS_(n)) whichare arranged to generate a voltage higher than the supply voltage(V_(dd)), whereby the stages (S) are arranged in serie and a requiredmultiplication factor (MF) of the charge pump (CP) is adjustable byactivating/deactivating a definable number of stages (S), whereby theswitches (SW_(n)) of each stage (S_(n)) are arranged in the same way. 2.Charge pump device as claimed in claim 1, whereby for a multiplicationfactor (MF) smaller than the maximal possible multiplication factor(MF_(max)) the stages (S) beginning from an input (IN) of the chargepump (CP) device will be deactivated.
 3. Charge pump device as claimedin claim 1, whereby a switch (SW₁) comprises a switch MP1 which isarranged between an input (IN) and an output (OUT) of the stage (S) ofthe charge pump device further two transistors MP2 and MP3 forcontrolling the isolated bulk of the switch (SW₁) and a fourthtransistor MP4 to charge a boot capacitor (C_(B)), whereby the bootcapacitor (C_(B)) is arranged for storing the charge to drive the gateof the switch (MP1) further comprises a gate switch control unit (GSU),whereby the gate switch control unit (GSU) is arranged to control thegate of the switch MP1.
 4. Charge pump device as claimed in claim 3,whereas the switch MP1 is preferably realized as isolated bulktransistor.
 5. Charge pump device as claimed in claim 3, containing alevel generation unit (LGU) for providing control signals for the gateswitch control unit (GSU).
 6. Charge pump device as claimed in claim 3,whereby the gate switch control unit (GSU) is foreseen to connect ordisconnect the gate of the switch MP1 transistor from the C_(B)capacitor.
 7. Charge pump device as claimed in claim 3, whereby the gateswitch control unit (GSU) is arranged to provide control signals to theswitch (SW) of the stage (S) in case of voltages below V_(dd). 8.Display driver (DD) for providing display information and voltages to adisplay unit (DU) with a charge pump (CP) device containing at least twostages (S), whereby a stage comprises a switch (SW_(N)) and a chargedevice (CS_(n)) which are arranged to generate a voltage higher than thesupply voltage (V_(dd)), whereby the stages (S) are arranged in serieand all stages (S) are realized in the same way and a requiredmultiplication factor is adjustable by activating/deactivating a certainnumber of stages (S).
 9. Display module (DM) having a display unit (DU)and a display driver (DD) with a charge pump (CP) device, containing atleast two stages (S), whereby a stage comprises a switch (SW_(n)) and acharge device (CS_(n)) which are arranged to generate a voltage higherthan the supply voltage (V_(dd)), whereby the stages (S) are arranged inserie and all stages (S) are realized in the same way and a requiredmultiplication factor (MF) is adjustable by activating/deactivating acertain number of stages (S).
 10. Telecom terminal having a displaymodule (DM), a display unit (DU) and a display driver (DD) with a chargepump (CP) device, containing at least two stages (S), whereby a stagecomprises a switch (SW_(n)) and a charge device (CS_(n)) which arearranged to generate a voltage higher than the supply voltage (V_(dd)),whereby the stages (S) are arranged in serie and all stages (S) arerealized in the same way and a required multiplication factor (MF) isadjustable by activating/deactivating a certain number of stages (S).